The authors provide a list of the alien fish species that have entered the Romanian ichthyofauna fauna and specify the year, place of signaling and the ecological impact of the respective species. Further on, the authors describe some behavioral aspects of the following species: Lepomis gibbosus (pumpkin seed sunfish), Carassius auratus gibelio (silver crucian carp) and Pseudorasbora parva (topmouth gudgeon).